1. Field of the Invention
This invention relates to a method and apparatus for controlling an induction motor which supplies power from power conversion means capable of changing both of its output frequency F.sub.1 and output voltage V.sub.1 , such as an inverter or a cycloconverter, to an induction motor which is rotating inertially, and reactuates the induction motor.
2. Description of the Prior Art
The necessity of reactuating an induction motor which is rotating inertially develops when instantaneous service interruption occurs.
Such a necessity occurs also when a load which has a large inertia and has been actuated by a commercial power source must be stopped quickly by changing the connection to the actuation by power conversion means capable of changing both of its output frequency and output voltage and by applying regenerative braking.
When power having a frequency remarkably different from the frequency of a residual voltage of an induction motor which is rotating inertially is applied to the induction motor, a large current flows through the induction motor and through the power conversion means. The power conversion means capable of withstanding such a large current is considerably expensive.
Japanese Patent Laid-Open No. 8250/1980 discloses a method which detects the frequency of this residual voltage, brings the initial frequency of the power to be applied from the power conversion means into agreement with this frequency and reactuates the induction motor.
On the other hand, Japanese Patent Laid-Open No. 129198/1982 proposes a method which detects the rotating speed of the induction motor by use of a speed generator, and assumes the frequency of the residual voltage of the induction motor from the output of the speed generator. In order to couple the speed generator to the induction motor, however, a specific work must be applied to the output shaft of the induction motor.
From this aspect, the system which directly detects the residual voltage is advantageous because counter-measures can be taken within a control circuit without any specific work for the induction motor. The drawback of this system, however, is that the residual voltage will become zero within a short period of time depending upon the condition of the load, although the induction motor is rotating inertially. If excessive a.c. power is applied to the induction motor during the intertial rotation when no residual voltage exists, a large load current flows, too.
When the primary side of the induction motor is kept open (equivalent to the cut-off of the power source), the residual voltage Vm on the primary side can be expressed as follows: ##EQU1## where T.sub.2 is a secondary time constant and is given by ##EQU2## M is mutual induction between the primary and secondary sides, r.sub.2 is secondary resistance, L.sub.2 is secondary inductance, .omega. is an angular velocity of rotor, and i.sub.20 is the initial value of a secondary current.
In eq. (1) described above, the angular velocity .omega. of rotation does not drop drastically under the condition that though the inertia of the load, that is, GD.sup.2 is great, the load itself is small. Therefore, ##EQU3## where V.sub.10 is an effective value of the residual voltage on the primary side immediately after opening of the primary side.
The period of the waveform Vm of the residual voltage at this time greatly depends upon the secondary time constant, and damps with a substantially equal period over a relatively long period as shown in FIG. 1.
On the other hand, under the condition that GD.sup.2 of the load is small but the load itself is great, the residual voltage Vm is given by: ##EQU4##
The waveform Vm of the residual voltage at this time is such as shown in FIG. 2, and both the period and the amplitude change drastically, and the waveform damps within a relatively short period.
As is obvious from the description given above, the residual time of the residual voltage greatly depends upon the greatness of the load, the greatness of the inertia of the load and the greatness of the secondary time constant. Even if the load is small and the inertia is great, the residual voltage can not be detected depending upon the length of the service interruption time if the secondary time constant is small, even though the induction motor is rotating inertially.